1. Field of the Invention
This invention relates to novel labeled conjugates for use in specific binding assays for an iodothyronine, particularly a thyroid hormone such as thyroxine, in a liquid medium such as serum.
The iodothyronines have the following general formula: ##STR2## wherein .beta..sup.1 and .beta..sup.2 are, independently, hydrogen or iodine. The principal iodothyronines of clinical interest are listed in Table 1 below.
TABLE 1 ______________________________________ Iodothyronine .beta..sup.1 .beta..sup.2 ______________________________________ 3,5,3',5'-tetraiodothyronine iodine iodine (thyroxine; T-4) 3,5,3'-triiodothyronine iodine hydrogen (liothyronine; T-3) 3,3',5'-triiodothyronine hydrogen iodine ("reverse" T-3) 3,3'-diiodothyronine hydrogen hydrogen ______________________________________
The quantitative determination of the concentration of the various iodothyronines, particularly the hormones T-3 and T-4, in serum and of the degree of saturation of the iodothyronine binding sites on the carrier protein thyroid binding globulin (TBG) are valuable aids in the diagnosis of the thyroid disorders.
2. Brief Description of the Prior Art
Various methodologies exist for the determination of iodothyronine concentrations in serum. A significant advance in iodothyronine assays was the development of the competitive protein binding assay by Murphy and Pattee, J. Clin. Endocrinol. Metab. 24:187(1964) in which radiolabeled iodothyronine competes with serum iodothyronine for binding to TBG. The development of specific antiserum for the various iodothyronines permitted radioimmunoassays to be devised in which radiolabeled and serum iodothyronine compete for binding to antibodies rather than to TBG. In both the competitive protein binding assay and the radioimmunoassay for an iodothyronine, the radiolabeled material consists of the native iodothyronine in which one or more of the iodine atoms are replaced by a radioactive iodine isotope, usually .sup.125 I.
Recently there have been developed non-radioisotopic specific binding assays which are applicable to the detection of various ligands including the iodothyronines. U.S. Pat. Nos. 4,043,872 and 4,040,907 describe a homogeneous enzyme-labeled binding assay for thyroxine and the enzyme-labeled thyroxine conjugate used therein. Other non-radioisotopic binding assays are described in German Offenlegungschriften Nos. 2,618,419 and 2,618,511 based on U.S. Ser. Nos. 667,982 and 667,996, filed on Mar. 18, 1976, and assigned to the present assignee, involving the use of particularly unique labeling substances, including coenzymes (e.g., FAD), chemiluminescent molecules, cyclic reactants, and cleavable fluorescent enzyme substrates. A further improvement in non-radioisotopic binding assays is offered in the U.S. patent application filed on even date herewith entitled "Specific Binding Assay with a Prosthetic Group as a Label Component" Ser. No. 917,961 and assigned to the present assignee, describing the use of organic prosthetic groups such as FAD as labeling substances in binding assays.